The two conventional propulsion systems for public transport buses are internal combustion, usually diesel, engines and direct current electric motors supplied with power from overhead trolley lines. An alternating current electric system is also available from Stromberg Industrial Systems of Helsinki, Finland. This is a trolley bus system in which the DC supplied is converted by a variable voltage, variable frequency, pulse width modulated inverter into an alternating current for driving a three phase AC traction motor. Another known system is the "dual mode" system using two separate drive systems, one electric and the other internal combustion. The use of two complete propulsion systems leads to numerous problems, including excessive weight, dual controls and the like.
Recently, a system has been developed that employs a diesel engine-alternator/rectifier system for supplying DC power to a Stromberg type AC electric propulsion system. A system of this sort, set up as a "dual mode" system with DC power coming from either the engine-alternator or overhead trolleys has been constructed as a prototype. With that system, the diesel engine and the main power inverter were both controlled according to an input from the operator through an accelerator pedal. The diesel engine was therefore operated according to the speed and power demanded by the operator and not according to the power requirements of the system to meet the operator's demands.
The present invention is concerned with improvements relating to this and other aspects of a vehicle of this last mentioned type.